1. The Field of the Invention
This application relates to optical receivers and, more particularly, for receivers including electronic dispersion compensation.
2. The Relevant Technology
In an electronic dispersion compensating (EDC) system, signals are received over a transmission channel, such as an optical fiber, from a transmitter. Optical signals are converted to electrical signals, such as by means of a photodiode. The electrical signals are supplied to an analog front end (AFE) of the EDC, which typically includes a clock recovery unit (CRU), a variable gain amplifier (VGA), and an analog to digital converter (ADC). The VGA both amplifies and applies a bias to the electrical signal prior to sampling by the ADC. The clock recovery unit detects bit transitions in the electrical signal in order to generate a clock signal in phase with the received signal. The ADC samples the electrical signal at a time offset relative to the recovered clock signal in order to generate a sampled signal. Each sample typically includes a multi-bit value, such as four, eight, or sixteen bits.
The samples are supplied to a maximum likelihood sequence estimator (MLSE) which examines a sequence of samples simultaneously to decode the information encoded in the transmitted signal. Multiple samples are examined simultaneously to enable equalization of intersymbol interference (ISI) caused by pulse spreading during transmission.
In some MLSEs, such as the MLSE disclosed in U.S. patent application Ser. No. 11/736,515, filed Apr. 17, 2007, which is incorporated herein by reference, a channel estimator is used to model the channel over which the signal is transmitted. More specifically, the channel estimator models intersymbol interference experienced by the transmitted signal. Accordingly, the channel estimator will output for a given multi bit sequence, an expected sampled value for a given bit in that sequence. For example, for the bit sequence 010, the model of the transmission channel may predict a sampled value of 14 (b1110), for the second bit of the bit sequence.
The channel estimates for some or all possible combinations of the multi bit sequence, e.g. 000, 001, 010 . . . , for a three bit sequence, are compared to the sampled values. The MLSE chooses a series of bit sequences such that a combined error metric of a sequence of sampled values relative to the estimates corresponding to the chosen series of bit sequences is at a minimum relative to other possible series of bit sequences. The series of bit sequences are decoded to yield a data word.
In a typical receiver system, the EDC may not receive information regarding the bit error rate (BER) of the decoded data relative to the originally transmitted data. Transmitted data may include parity bits that are analyzed to determine whether data has been correctly decoded. However, because the EDC does not receive this information it is not able to tune its operation such that the BER is reduced.
In view of the foregoing, it would be an advancement in the art to provide a system and method for reducing the BER of a received signal by tuning an EDC without providing the actual BER to the EDC.